Telomeres, the nucleoprotein structures that cap chromosome ends, are maintained by telomerase, a multi-subunit enzyme complex. In settings of insufficient telomerase, including in aging human tissues, telomeres shorten with cell division. Telomerase subunits are mutated in certain human genetic diseases, such as dyskeratosis congenita, in which telomere shortening is accelerated and tissue progenitor cells are dysfunctional. The profound impairment of tissue progenitor cell function caused by telomere shortening in both human genetic syndromes and in knockout mouse models provides strong support for the hypothesis that telomere shortening in aging humans contributes to aspects of the aging phenotype. However, the precise mechanisms that underlie telomere shortening with aging and the specific approaches for blunting or reversing telomere shortening as a therapeutic strategy remain very poorly understood. Defining why telomeres shorten with advancing age and developing therapeutics to prevent such telomere shortening will require a much more complete understanding of telomerase. Although telomerase enzyme extracted from human cancer cells behaves as a very large complex, only three components of the telomerase holoenzyme were identified until recently: TERT, the telomerase reverse transcriptase, TERC, the telomerase RNA component, and dyskerin, a TERC-binding protein. To address this critical area in aging research, we have purified telomerase complexes from human cells and identified new telomerase-associated proteins by mass spectrometry. Through this approach, we identified a new component of the telomerase holoenzyme, termed TCAB1, for Telomerase Cajal Body Protein 1. TCAB1 associates with all active telomerase enzyme and associates with all TERC in human cell extracts. Importantly, TCAB1 is specifically found in Cajal bodies, subnuclear foci that serve as sites of ribonucleoprotein complex modification or assembly. Cajal Bodies were recently shown to be sites of telomerase localization and TCAB1 is the first Cajal body-specific protein component of the holoenzyme. Depletion of TCAB1 in human cells using RNA interference prevents telomerase from localizing in Cajal bodies and leads to profound telomere shortening. Thus, TCAB1 is a unique and essential telomerase component required for telomerase trafficking and telomere synthesis. We plan the following: (1) To study the biochemistry of TCAB1 in the telomerase complex and in scaRNA complexes (2) To study the requirement for TCAB1 in telomerase function through genetic approaches in cultured cells and through analysis of the TCAB1 protein complex (3) To understand the role of TCAB1 in telomerase function and stem cell regulation in vivo.